Single-component, room-temperature vulcanizable compositions are known in the prior art which vulcanize or cure to rubbery solids at room temperature. Examples of these compositions are found in U.S. Pat. No. 3,689,454; 3,619,255; 3,647,725; 3,705,120; and copending Ser. No. 417,787, filed Nov. 21, 1973 all of which are hereby incorporated by reference.
The prior art method of preparing room temperature vulcanizable compositions (RTV) has been by using a two-part procedure of producing a base compound of diorganopolysiloxane and a cross-linking agent. Thereafter, a separate catalyzation step is carried out in another piece of equipment. In particular, low modulus, one-package, room-temperature vulcanizable elastomers have been prepared in this manner and difficulty has been encountered in the production of these compositions. The abbreviation of RTV as used herein means a room-temperature vulcanizable material.
The chemical reactions, which form the curable low-modulus RTV, result in the elaboration of substantial amounts of heat which can result in a bubbled product due to the vaporization of the byproduct methanol (b.p. = 64.7.degree.C). Also high shear mixing is required during a high viscosity rise which occurs during the preparation of the curable product. These factors have resulted in a batch type manufacturing process using different apparatus for different processing steps.
It has now been found that a low modulus curable RTV composition may be prepared by passing the organodipolysiloxane, the cross-linking agent and the catalyst through a devolatilizing extruder. This process allows for continuous production and facilitates the incorporation of compounding ingredients at precise points in the production stream so as to obtain the desired degree of dispersion and chemical reaction. Surprisingly, it has been found that the use of viscosity minimizing agents is substantially reduced or eliminated in this process and, therefore, the product has an extended shelf life of 18 months as compared to the product of the prior art process which has a shelf life of about 8 months. Accordingly, it is a primary object of this invention to provide an improved process for preparing a curable low modulus RTV composition.
It is also an object of this invention to provide a continuous process for the production of a curable catalyzed RTV composition.
It is also an object of this invention to provide a process for the production of a curable, catalyzed RTV composition which may be carried out without a viscosity minimizing agent.
It is also an object of this invention to provide an improved process for the production of a curable, catalyzed RTV composition which rapidly deaerate and devolatilizes the composition as it is being blended.